Upon sub-monolayer Ge deposition on the 2×n reconstructed SiGe alloy wetting layer at room temperature, the predominant adspecies observed are so-called addimer chain structures (ADCSs). Polarity-switching scanning tunneling microscopy (STM) is used to study ADCSs, as they only appear in empty-state images. ADCSs are dilute structures comprising type-C addimers that reside in neighboring troughs and extend along all equivalent <130> directions of the surface, giving rise to a zigzagged morphology. ADCSs exhibit localized movements, wherein their rearrangements are strongly coupled with the structure of the underlying substrate dimers. At elevated temperatures, ADCSs are observed to transition to compact epitaxial segments of the next atomic layer, indicating their metastability. By measuring ADCS transition rates over the temperature range of 90–150°C, an activation energy of 0.7 ± 0.2 eV and an associated prefactor of 5 × 104±2 s–1 are computed. Both of these kinetic values are quite low as compared to typical surface diffusion phenomena, and we suggest that the transition mechanism is likely complex and multi-bodied, requiring concerted rearrangements of the addimer chain with underlying substrate dimers.
To achieve chemical contrast between Si and Ge in the alloy wetting layer, preliminary investigations using scanning tunneling spectroscopy were conducted. A variety of surfaces with increasing Ge compositions were investigated. It is shown that for low Ge compositions of about 0.1 monolayers (ML) or below, low-bias empty-state STM imaging is useful in determining the locations of Ge intermixing. At higher compositions (0.25–0.5 ML), considerable contrast and structure in the conductance images is observed that corresponds to structure in STM images and the Ge content of the surface. For compositions of 1.5 ML, which give rise to a fully-developed 2×n reconstruction, the only structure in conductance images corresponds to the locations of the dimer vacancy lines, perhaps indicating sub-surface Ge depletion zones.